1. Field of the invention
The present invention relates to an apparatus for scrubbing exhaust gases, and more particularly to a scrubber for removing waste gases from exhaust generated when rocket motors are test fired, in a manner that is both environmentally compliant and economically acceptable.
2. Description of the Background
The United States Department of Defense test fires thousands of rocket motors each year for lot acceptance testing, rocket motor development, and surveillance testing of fleet returned motors. Rockets range in size up to 1,000,000 pounds (propellant weight). Test firing these rockets produces tons of gaseous products every year, most of which is of concern to the Environmental Protection Agency.
When rocket motors are fired during static testing procedures, most of the rocket propellant is burned and the combustion products are exhausted from the rocket motor through a motor nozzle. Rocket motor emissions are a point source of pollution as they contain toxic compounds and heavy metals, many of which are on the Environmental Protection Agency's List of Controlled Substances. The combustion products include gaseous HCl, Al2 O3, CO and Pb, although the combustion products will vary depending upon propellant composition. Different propellants have different combustion products, although most are similar in nature.
There are some devices known as “scrubbers” for cleaning waste gas emissions in other contexts. Specifically, prior art “scrubbers” have addressed the cleaning of effluents generated from industrial waste streams. A process for treating waste gases produced from an industrial process is shown, for example, in U.S. Pat. No. 4,990,317 to Mak, et al. The process comprises contacting the waste gas in a reaction unit with an alkaline brine solution under conditions such that the HCl reacts with the alkali to form a chloride salt. The gas is then conducted to a scrubber for further treatment and removal of any residual HCl. Finally, according to the process, a demister apparatus gives the gas a final wash with fresh water before being released into the atmosphere. The invention includes a process for the treatment of waste gas containing HCl wherein the gas is at an elevated temperature. The Mak '317 process is a continuous flow process and is not suitable for use with rocket motors. The process is not designed to capture and clean gases emitted at the high velocities attained by rocket motor exhaust gases.
U.S. Pat. No. 5,955,037 to Holst, et al. illustrates another system for the treatment of industrial effluent gases produced in semiconductor manufacturing processes. The device shows a treatment chamber for acid gas and particulate removal, an oxidizer exhaust gas quenching unit, an acid gas scrubber, and a flow-inducing means such as a fan for active motive flow of the gas through the system.
U.S. Pat. No. 5,122,169 to Schumacher, et al. discloses yet another apparatus for the removal of pollutant gases and liquid droplets from a gas stream generated from manufacturing processes. The system illustrates a scrubbing apparatus, a spray injection system for spraying scrubbing fluid in droplet form through the gas stream, an absorption screen, and a diffusion filter for removal of residual liquid droplets.
The foregoing and other commercial” scrubbers on the market are mainly for normal industrial conditions in environments of reasonably uniform operating conditions. They do not contemplate the temperature and velocity extremes present in the exhaust of a rocket motor. Scrubbing rocket motor gases poses unique challenges because the gases emitted are very hot, reaching temperatures of 3500 degrees Fahrenheit, and are expelled at very high velocities, averaging up to 2900 ft/sec. Furthermore, in the context of a static test of a rocket motor, care must be taken so that the scrubber does not influence the thrust and performance capabilities of the rocket being tested. Furthermore most industrial scrubbers are not concerned with avoiding any performance impact on a rocket motor being fired into the scrubber. The only reason for performance testing a rocket motor is to get accurate performance data. If the scrubber influences the performance data, there is no reason to perform the test.
There has been one known effort to develop a rocket exhaust scrubber at Edwards Air Force Base. However, this scrubber was very intrusive and did not measure the thrust and performance capabilities of the rocket being tested. Significantly, the Edwards scrubber employed a static ventilation flow path which causes a pressure rise at the end of firing when the rocket motor exhaust gas velocity head is converted into static pressure, which induces back pressure on the rocket motor providing an less accurate thrust measurement.
In addition, the Edwards scrubber caused a potential for ignition of the H2 and CO in the rocket exhaust. After a rocket motor fires there is nearly always a flammable quantity of these gases present, and when the exhaust gases combine/mix with the ambient air already in the scrubbed or added air, a serious potential for ignition exists. The Edwards scrubber was also prone to overpressure, and secondary combustion of the exhaust gases. These pressures and temperatures caused problems as it exceeded the physical limits of the scrubber. Also, the Edwards scrubber made no provisions for recycling the spray solution.
Aside from the foregoing, there also are a number of existing high altitude test chambers at Arnold Air Force Base that capture and provide some “washing” of gases with cooling water, but these are high altitude testing chambers and are not well-suited for production testing of rocket motors.
There currently are no known existing rocket motor exhaust scrubbers capable of accurately, safely, and economically measuring thrust in an environmentally acceptable manner.
Accordingly, it would be advantageous to provide a device that collects and scrubs rocket motor exhaust during test firing to remove toxic chemicals, heavy metals and particulates from the waste gas stream. It would further be advantageous that the device capture and clean the exhaust stream without impeding the thrust and performance capabilities of the rocket motor being tested. Additionally, it would be advantageous that the device capture and cool the gases that are emitted at the high velocities and elevated temperatures characteristic of rocket motor exhausts.